Hospital billed me $3,200 AFTER insurance - anyone else deal with this?

So i had an outpatient procedure done in January. My insurance said the facility was in-network, the surgeon was in-network, everything should be covered at 80/20 after deductible. I paid my portion at the time of service - about $340.

Fast forward two months and I get a bill for $3,200 from the hospital. Called my insurance and they said an assistant surgeon was "out-of-network" and I'm responsible for the full amount. I had no idea there even was an assistant surgeon and I definitely never consented to an out-of-network provider.

Is this the surprise billing thing I've heard about? I thought there was a law against this now. Has anyone successfully fought one of these? The hospital's billing department keeps saying it's not their problem and to deal with my insurance, and my insurance says it's not their problem either.